Abstract
Aerosol mass efficiencies for extinction, scattering, and absorption are important parameters to understand aerosol optical properties. Although the mass efficiency is functions of the refractive index and particle size distribution, due to the complexity of the efficiency, mass efficiency parameters are usually regarded as a size independent and assumed to depend mainly on the chemical composition of aerosols. In this study, we calculated the mass efficiencies of polydispersed aerosols based on different aerosol types. An analytical approach to the approximated formula of the mass efficiency of each chemical species was developed and evaluated by fitting the results to those of the Mie theory that calculated the optical properties of chemical species based on the refractive index and size. We used the lognormal size distributions and external mixture approximations that represent the polydispersity of aerosol particles. Size ranges of 0.5–2.5 µm in the geometric mean diameter were considered for five different chemical species. The parameters of fitting curves were generalized for polydispersed aerosols as functions of the geometric mean diameter and the geometric standard deviation. The results of the newly developed analytic approach showed a good agreement with those of the Mie theory. The proposed approach provides an effective means to estimate the mass extinction efficiency of polydispersed multi-component aerosols.
Highlights
Atmospheric aerosols play an important role in regulating earth’s radiation budget and this regulation depends strongly on their optical properties
We have considered different density for different aerosol species
This variation of optical properties is closely related to aerosol size distributions because extinction coefficients vary with the aerosol size
Summary
Atmospheric aerosols play an important role in regulating earth’s radiation budget and this regulation depends strongly on their optical properties. Light scattering and absorption characteristics of atmospheric aerosols, which affect their optical properties, depend on several factors, such as the wavelength of the incident light, the size distribution, shape, chemical composition and the mixing state of aerosol particles, and relative humidity (RH) that determines the aerosol water content (Pilinis et al, 1995). Bscat and babs (in Mm–1) are the reconstructed scattering and absorption coefficients, respectively; f(RH) is the relative humidity scattering enhancement factor, which is the ratio between dry and wet scattering as a function of RH; fAS(RH) and fss(RH) are the enhancement factors for ammonium sulfate [(NH4)2(SO)4] and sea salt (NaCl), respectively; C[(NH4)2SO4] and C[NH4NO3] are fully neutralized ammonium sulfate and ammonium nitrate (NH4NO3) mass concentrations in μg m–3, respectively; C[OMC], C[SOIL], C[CM], and C[SS] are the mass concentrations of organic carbon, soil, coarse mass (CM), and fine sea salt (SS), respectively. Mass absorption efficiency (MAE) is the ratio between the aerosol absorption coefficient and the aerosol mass concentration in a unit volume of air (Malm et al, 2000; Hand and Malm, 2007)
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